1. Field of the Invention
This invention relates to a magnetic recording medium used for writing and reading information. More particularly, this invention relates to a magnetic recording medium that has an improved crystallinity of a magnetic recording layer to improve an S/N ratio, and has an improved thermal stability to provide high-density recording. The present invention relates further to a production method of such a magnetic recording medium and a magnetic storage apparatus using the magnetic recording medium, particularly a magnetic disk apparatus.
2. Description of the Related Art
The requirement for high-density recording has increased in magnetic disk apparatuses, used for external storage for computers, with the development of information processing technologies. Speaking more concretely, a magneto-resistive head using a magneto-resistive element, the electric resistance of which changes in accordance with the intensity of a magnetic field, that is, an MR head (Magneto-Resistive) head has been used recently in place of a conventional wired type of inductive thin film magnetic head. The MR head utilizes the magneto-resistance effect, that is, the change of the electric resistance of a magnetic substance due to an external magnetic field, for reading signals on a recording medium. The MR head has features in that it can provide an extremely greater reading output width than conventional inductive thin film magnetic heads, its inductance is small and a large S/N ratio can be expected. An AMR (Anisotropic Magneto-Resistive) head utilizing an anisotropic magneto-resistance effect, a GMR (Giant Magneto-Resistive) head utilizing a giant magneto-resistance effect and a spin valve GMR, as a practical type of GMR, have been used as MR heads.
To satisfy the requirement for high-density recording, further, the improvement of characteristics capable of coping with the MR head, the AMR head or the GMR head (inclusive of the spin valve head) has been required from the magnetic recording medium used for writing and reading the information in the magnetic disk apparatus, too. In other words, a magnetic recording medium having a low noise but a high S/N ratio and capable of high-density recording has been required.
Conventional magnetic recording media have generally been produced by forming an underlayer from chromium or its alloys on a non-magnetic substrate such as an aluminum substrate, and then forming a magnetic recording layer from an alloy containing cobalt as its principal component, such as a CoCrTa alloy or a CoCrPt alloy, on the underlayer. In such recording media, the (11-20) plane of the recording layer grows in parallel with the (002) plane of the Cr type layer used as the underlayer due to coincidence of the lattice gap. In consequence, the magnetic recording layer (Co type layer) has in-plane anisotropy, and can improve the in-plane write characteristics. However, the Co type layer (which will be called the xe2x80x9cinitial stage magnetic recording layerxe2x80x9d or the xe2x80x9cinitial growth layerxe2x80x9d in this specification) formed at the initial stage of the film formation has low crystallinity and results in a low S/N ratio.
To improve the crystallinity of the initial stage magnetic recording layer, Japanese Unexamined Patent Publication (Kokai) No. 8-329444 discloses a magnetic recording medium characterized in that an intermediate layer comprising a cobalt alloy having a hcp structure is sandwiched between a cobalt magnetic layer on a non-magnetic substrate and an underlayer comprising cobalt as the principal component in a magnetic recording medium having the magnetic layer on the non-magnetic substrate. In this magnetic recording medium, the non-magnetic hcp-Co type layer is allowed to grow as the initial stage magnetic recording layer to improve the crystallinity of the Co magnetic layer grown on the hcp-Co type layer.
Another method is based on the technical concept that it is effective, for lowering the noise level of magnetic recording media, to reduce the magnetic particle size to a very small size, to make the particle size uniform and to cut off the magnetic interaction among the magnetic particles. This method adds a suitable additive on the basis of this concept. For example, in a magnetic recording medium comprising a non-magnetic metal underlayer, a magnetic layer and a protective layer that are disposed on a non-magnetic substrate layer, Japanese Unexamined Patent Publication (Kokai) No. 7-50008 describes a magnetic recording medium wherein the magnetic layer comprises 60 to 80 at% of Co, 5 to 20 at% of Cr, 1 to 20 at% of Pt and 0.5 to 6 at% of at least one of Nb, Hf, W, Ti and Ta. In other words, this magnetic recording medium attempts to attain a low noise by adding at least one of Nb, Hf, W, Ti and Ta to the magnetic layer. In such a recording medium, however, thermal unstability of recording magnetization, that results from the decrease of the magnetic particle size and reduction of the magnetic interaction among the magnetic particles, becomes a new problem.
The conventional technologies explained above may be summarized as follows. In order to accomplish high-density recording and a low noise level in a magnetic recording medium, it is essentially necessary to reduce the sizes of the magnetic particles in the magnetic recording layer, to make the particle sizes uniform and to reduce the magnetic interaction among the magnetic particles. However, recording magnetization becomes unavoidably unstable in this case, and measures must be taken to secure thermal stability of recording magnetization.
The inventors of this invention have conducted intensive studies to secure thermal stability of recording magnetization. As a result, the present inventors acquired the following idea, as described in Japanese Unexamined Patent Publication (Kokai) No. 11-96534. In other words, the present inventors have invented a magnetic recording medium comprising a first underlayer made of a non-magnetic material consisting of chromium as the principal component, a second underlayer made of an anti-ferromagnetic material of a body-centered cubic structure, having a Neel temperature of at least 60xc2x0 C., of the type such that when the crystal lattice of this material is compared with the crystal lattice of the body-centered cubic structure of the first underlayer, one of the sides of the lattice of the (100) plane of the former is substantially in conformity with the length of the diagonal of the lattice of the (100) plane of the latter, hence, epitaxial growth can be done on the first underlayer, and a magnetic layer made of a ferromagnetic material consisting of cobalt as the principal component, each layer being disposed serially on a non-magnetic substrate. This magnetic recording medium uses the anti-ferromagnetic material for the second underlayer keeping contact with the magnetic layer. Consequently, this medium provides improved thermal stability by utilizing the exchange interaction between the anti-ferromagnetic underlayer and the ferromagnetic magnetic layer.
In this magnetic recording medium, however, the anti-ferromagnetic underlayer and the ferromagnetic magnetic layer must be in direct contact with each other. Therefore, the improvement of crystallinity and a high S/N ratio cannot be accomplished simultaneously by employing the initial stage growth layer (the magnetic recording layer of the initial stage) having the hcp structure that is described in Japanese Unexamined Patent Publication (Kokai) No. 8-329444 described already.
The present invention is therefore directed to solve the problems of the prior art technologies described above. It is the first object of the present invention to provide a magnetic recording medium that can simutaneously provide an improvement in the S/N ratio and the thermal stability of recording magnetization, and is suitable for high-density recording.
It is the second object of the present invention to provide a production method of such an excellent magnetic recording medium.
It is the third object of the present invention to provide a magnetic storage apparatus using the magnetic recording medium of the present invention.
The above and other objects of the present invention will be easily understood from the following detailed explanation.
According to one aspect of the present invention, there is provided a magnetic recording medium including an underlayer made of an anti-ferromagnetic material, and a magnetic recording layer made of a magnetic material consisting of cabalt as the principal component, that are disposed serially on a non-magnetic substrate, wherein an intermediate layer made of a ferromagnetic material is further sandwiched between the underlayer and the magnetic recording layer.
According to another aspect of the present invention, there is provided a method of producing a magnetic recording medium including an underlayer and a magnetic layer on a non-magnetic substrate, which method comprises forming the underlayer from an anti-ferromagnetic material on the non-magnetic substrate, forming the intermediate layer from a ferromagnetic material, and forming the magnetic recording layer from a magnetic material consisting of cobalt as the principal component on the intermediate layer, whereby each of these layers is formed by sputtering.
According to still another object of the present invention, there is provided a magnetic storage apparatus including a write head portion for writing information and a read head portion for reading information in a magnetic recording medium, wherein the magnetic recording medium is a magnetic recording medium including an underlayer made of an anti-ferromagnetic material, an intermediate layer made of a ferromagnetic material and a magnetic recording layer made of a magnetic material consisting of cobalt as the principal component that are serially disposed on a non-magnetic substrate, wherein the write head portion is equipped with a magneto-resistive head.